1. Technical Field
The present invention relates to a method of forming a copper-Invar-copper (CIC) laminate having an intermetallic layer of negligible thickness, and a structure associated with the CIC laminate.
2. Related Art
A CIC laminate includes a block of Invar sandwiched between two layers of copper. A CIC laminate of a specified thickness and having desired mechanical properties (e.g., ductility) may be formed by a process that includes cladding copper and Invar to form a layered CIC laminate, followed by rolling and annealing the layered CIC laminate to form the working CIC laminate. The rolling step starts with a feed stock of a CIC sheet and rolls the CIC sheet to the desired smaller thickness. The annealing step anneals the rolled CIC sheet at a temperature typically in a range of 760° C. to 870° C., wherein the annealing reduces the grain size in the CIC laminate. The rolling and annealing is done multiple times to achieve the desired thickness and mechanical properties of the CIC laminate. A disadvantage of the preceding process is that a copper/Invar thickness ratio in the formed CIC laminate cannot be arbitrary, because there are only several standard feed stocks of CIC, with each standard feed stock having a particular CIC/Invar thickness ratio. A second disadvantage is that a copper layer of the CIC laminate may have undesired thickness variations due to rolling defects. A third disadvantage is that the formed CIC laminate includes an undesired intermetallic layer between each copper/Invar interface, wherein the intermetallic layer has a thickness typically between 1.5 and 8 microns. The intermetallic layer, which is generated as a consequence of the temperature of the annealing, includes constituents of the CIC laminate, namely copper, iron, nickel. Thus an intermetallic layer of a CIC laminate is defined herein as a layer between a copper/Invar interface of the CIC laminate, such that the intermetallic layer includes constituents of the CIC laminate, namely copper, iron, nickel. FIG. 1 illustrates a CIC layer 300 thus formed, with an Invar layer 310 between a copper layer 321 and a copper layer 322, wherein an intermetallic layer 331 interfaces the Invar layer 310 and the copper layer 321, and wherein an intermetallic layer 332 interfaces the Invar layer 310 and the copper layer 322. Unfortunately, mechanical or laser drilling of a hole through the intermetallic layer within the formed CIC structure can generate debris from drilled material of the intermetallic layer. The generated intermetallic debris may be difficult to remove. Additionally, the debris removal by such methods as acid cleaning may result in preferential etchback of the copper or Invar in a direction normal to the surface that circumscribes the hole. The preferential etchback would result in undesired void formation within the CIC laminate adjacent to copper plating subsequently formed over the surface of the hole. If the intermetallic layer thickness were reduced or eliminated, the aforementioned problems relating to the intermetallic layer would be mitigated.
Thus, a there is a need for a method for forming a CIC laminate such that: the copper/Invar thickness ratio can have any desired value within a continuous range of values, the thickness of each copper layer is uniform, and the intermetallic layer is either eliminated or reduced in thickness.
After a CIC layer is formed according to the related art and becomes an internal layer within a dielectric substrate, drilling a through hole through the dielectric substrate generates a porous “metal skin” over a CIC surface of the through hole. The predominant constituents of the metal skin are metallic debris derived from the CIC laminate, such as copper, iron, and nickel debris, and alloys thereof. Unfortunately, a porosity of the metal skin allows processing chemicals, such as acid solutions, to become trapped between the metal skin and the CIC surface covered by the metal skin. As a consequence of the metal skin, a pocket void may form within the Invar layer of the CIC laminate due to galvanic action on a galvanic cell comprising a copper layer of the CIC laminate, the block of Invar of the CIC laminate, and the acid solution. Thus, a method is needed to form a dielectric substrate having CIC layer and a hole therethrough, wherein a metal skin is not generated.